1. Field of the Invention
The present invention relates to nonvolatile memory devices and, more particularly, to controlling access to memory cells within nonvolatile memory devices.
2. Description of the Related Art
Some types of nonvolatile memory devices use resistance materials, such as resistive random access memory (RRAM) devices, magnetic random access memory (MRAM) devices, and phase-change random access memory (PRAM) devices. These nonvolatile memory devices can store data using changes in the state of a phase-change material such as a chalcogenide alloy (in the case of PRAM devices), changes in the resistance of a variable-resistance material (in the case of RRAM devices), or changes in the resistance of a magnetic tunnel junction (MTJ) film in accordance with the magnetization state of a ferromagnetic material (in the case of MRAM devices). In contrast, dynamic random access memory (DRAM) devices or flash memory devices store data using electric charges.
In the case of PRAM devices, in particular, a phase-change material is controllably changed between crystalline or amorphous states through controlled heating and cooling cycles. The resistance of a phase-change material in a crystalline state is high, whereas the resistance of a phase-change material in an amorphous state is low. Thus, a phase-change material in a crystalline state may be defined as set data or a binary value of 0 and a phase-change material in an amorphous phase may be defined as reset data or a binary value of 1.